The present invention relates to apparatus for electronically testing printed circuit boards and of the type including a plurality of contact elements located in a contact array plane, the contact elements being connected to an electronic control and test means and adapted to be connected through longitudinally rigid test pins to contact positions of a connection carrier or circuit board to be tested, and the contact elements being mounted to yield resiliently and being supported against a contacting pressure applied during a test.
Because of increasing commercial pressure for miniaturization and also because of the concomitant reductions in manufacturing expenses, producers all over the world have begun providing electronic modules on the basis of circuit boards having connection sites or contact pads in an extremely fine grid (1/20 to 1/100 inch grid) and utilizing SMD technology, with component leads that are not inserted through bores (possibly through-contacted) in multi-layer circuit boards but that are connected to contact pads on the component placement side of each board.
Also, manufacturers have realized that bare boards should be tested for functionality prior to component placement to ensure that there are not more or not less, but precisely the number of connections required. As a result, the manufacturers of circuit board testing apparatus must of necessity offer equipment now which enables almost any size and configuration of printed circuit boards having contact pads on an extremely find grid (1/20 to 1/100 inch grid) to be tested without difficulties.
German Patent No. 33 40 180, corresponding to U.S. Pat. No. 4,674,006, discloses a contact array assembly for computer-controlled circuit board testing apparatus using the 1/10 inch grid in which the contact array is subdivided to define contact array sections each removably supported by elongated supporting struts on a base plate. The space so created is used for accommodating the electronic components associated with each contact array section. These components are connected through a plug-type connection with a two-dimensional control circuit provided on a base plate. These sections (referred to as "contact array modules") are identically constructed and exchangeable for each other at any position of the base plate. This design concept results in circuit board testing apparatus comprising a fairly large basic contact array (having e.g. 256 contacts in each of the X and Y directions), yet operable with a very small number of electronic modules, the number of which may be increased as needed without problems.
The desire underlying the present invention is to realize that same basic concept for apparatus operating with an extremely fine grid (1/20 to 1/100 inch grid). In the past, so-called "reduction adaptors" as described in German Pat. No. 33 40 179, corresponding to U.S. Pat. No. 4,614,386, were used to reduce the up to 64,000 contact positions of the initial 1/10 inch grid in the X and Y directions of the contact array to a 1/20 inch grid. However, to obtain such reduction, the maximum permissible circuit board dimensions had to be reduced by 50 percent in either direction. As a result, realization on the 1/20 inch grid of the circuit board testing apparatus of German Pat. No. 33 40 180 appears to be constrained by the limits inherent in miniaturization itself, as will be explained in detail hereinafter. It should be noted that the same limits apply regarding expenditures. In circuit board testing apparatus having a contact array assembly as proposed in German Pat. No. 33 40 180, contact between the connection points or pads on the circuit board under test and the contact elements of the contact array assembly is made by means of test needles each having a tip telescoping resiliently in the longitudinal direction thereof. In the case of the conventional 1/10 inch grid, these contact needles are relatively simple and inexpensive to fabricate. Problems arise, however, if the contact spacing is reduced to 1/20 inch or less, since in such case the test needles cannot have a diameter greater than 0.8 mm. Test pins so thin will buckle and be damaged beyond repair under even the slightest of transverse forces. Besides, resiliently telescoping test pins of this kind are of necessity very complex mechanically so that their manufacturing costs may cause problems, given the large number of such test pins required. Where the previous 1/10 inch grid comprised a maximum of 64,000 contact positions and required a corresponding number of test pins, the 1/20 inch grid results in up to 256,000 contact positions within the same external dimensions of the contact array. Quite obviously, as very large numbers of test pins may be necessary, the cost therefor may be considerable and decide the potential user against purchasing the equipment. Thus, for using the principle proposed in German Pat. No. 33 40 180 where applied to an extremely fine contact grid of 1/20 inch or less, it would be necessary to provide means and structure as simple in construction and as inexpensive in fabrication as possible.
German Utility Model No. 85 34 841.4 of Feb. 20, 1986, corresponding to U.S. patent application Ser. No. 886,639 filed July 15, 1986, proposes the use of uncontoured test pins which are longitudinally rigid and do not have resiliently telescoping contact tips, particularly in case localized connection site densities on the circuit boards are higher than the average connection site density on the 1/10 inch fundamental grid of the contact array of the circuit board test equipment. Since rigid uncontoured contact pins readily can be made to have a rather small diameter, allowing their use at connection site densities higher (at least locally) than the fundamental grid of the circuit board testing equipment without creating a serious risk of the individual test pins shorting against each other, and since uncontoured rigid test pins of this nature are rather inexpensive to fabricate, the use of rigid test pins in the testing of circuit boards laid out on a 1/20 inch grid appears to be obvious initially. It should be kept in mind, however, that the "Flexadapter" approach proposed in German Utility Model No. 85 34 841.4 uses a so-called "active fundamental contact array" which provides for length compensation of all of the rigid test pins used and thus for reliable contact between each one of the test pins and the object under test, such as a printed circuit board, a ceramic connector support or a flexible circuit carrier. Such length compensation is necessary to compensate for possible bending of the rigid test pins and for variable thicknesses of the printed circuit boards, and in order to ensure sufficient contact pressure. An active contact array of this nature is formed by the resilient portions of the contact pins being transferred to the fundamental contact array of the circuit board testing board testing equipment utilized through the use of short contact pins in the form of sleeve-shaped members having an end in the shape of a contact tip and another end internally tapered, each pin being supported by a spring inside the sleeve and provided to receive one end of the rigid test pin. As a result, the active contact array assembly comprises a plurality of short internally tapered test pins corresponding in number to the connection sites to be accommodated and housed in suitable structure above the contact array proper of the circuit board testing apparatus. As a consequence, these internally tapered test pins, being expensive to make, do not relieve the problem of elevated manufacturing expenses. Also, it is most difficult, if not impossible, to reduce sleeve-like test pins having a compression spring therein to a diameter on the order of 0.8 mm, as long as springs so thin and weak are supposed to create sufficient contact pressure, and material strength considerations prohibit a reduction of sleeve wall thickness to less than 0.2 mm. As a result, the problems created by an active contact array of this nature on a 1/20 inch grid ultimately would be prohibitive.